Pressure sensing in implantable medical devices

ABSTRACT

An implantable medical device for delivering a therapeutic substance to a delivery site in a patient. A reservoir holds a supply of the fluid therapeutic substance. A catheter has a proximal end, a delivery region and a lumen extending from the proximal end to the delivery region. The proximal end of the catheter is operatively coupled to the reservoir. The delivery region of the catheter is adapted to be placed proximate the delivery site in the patient. The therapeutic substance is adapted to be delivered through the lumen to the patient. A sensing device is operatively coupled with the lumen of the catheter being capable of detecting a pressure of the therapeutic substance in the lumen. A controller is operatively coupled to the sensing device, the controller being capable of taking an action in response to the pressure in the lumen.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/836,115, filed Apr. 30, 2004 and claims priority therefrom,

FIELD OF THE INVENTION

This application is a continuation of The present invention relatesgenerally to pressure sensing in implantable medical devices and, moreparticularly, to pressure sensing in implantable medical devicesdelivering a therapeutic substance to a patient.

BACKGROUND OF THE INVENTION

Implantable drug delivery or infusion devices and/or systems arecommonly used, for example when chronic administration of apharmaceutically active agent or therapeutic substance to a patient isrequired. An implantable infusion pump-catheter delivery system may bepreferred when it is important to deliver the agent to a specific siteor when the agent must be administered to spaced sites in tightlycontrolled, yet minute dosages.

Typically, an implantable therapeutic substance delivery device has areservoir, for holding a supply of therapeutic substance awaitingdelivery to a delivery site in the patient. A pump may be fluidlycoupled to the reservoir for creating fluidic pressure to facilitatedelivery of the therapeutic substance to the patient. A catheterprovides a pathway for delivering the therapeutic substance to thedelivery site in the patient.

All Darts of the therapeutic substance delivery device/system need tooperate adequately to ensure proper functioning of the device/system.While perhaps the least complex, catheters can have and can developoperational problems.

Sometimes catheters in such delivery systems can become obstructed orclogged. A partial or complete blockage could prevent the therapeuticsubstance from reaching the delivery site in the patient or, in the caseof a partial obstruction, could prevent an adequate supply of thetherapeutic substance from reaching the delivery site in the patient.

Catheters can also leak due to cuts, tears, etc. A leak, small or large,can also prevent the therapeutic substance from reaching the deliverysite in the patient. A leak can result in a double problem. In additionto the lack of therapeutic substance supplied to the delivery site ofthe patient, the therapeutic substance could be dispersed elsewhere inthe body of the patient which may create further issues

When catheters become clogged or leak and the infusion pump continues todeliver drug, a patient's well being may be placed in danger.

However, it has been difficult to detect the malfunction of a catheter.For example, if the catheter has a leakage, the implantable drugdelivery device could continue to delivery therapeutic substance andthere may be no way to know that the therapeutic substance was notreaching the desired delivery site. The patient may not receive thebenefit of the therapeutic substance but might not know why. As anotherexample, if the catheter has an obstruction, the implantable drugdelivery device might cease to deliver the therapeutic substance. But itmay be difficult to know why the failure occurred. The failure todeliver might have been caused by other factors, such as power failure,pump failure or an empty reservoir.

If a catheter malfunctions, it is desirable to know so that appropriatecorrective action can be taken.

BRIEF SUMMARY OF THE INVENTION

The present invention can detect a malfunction in a catheter and takeappropriate action if and when the malfunction occurs. By the sensing ofpressure in the lumen of the catheter, cuts and leaks might result inlower than normal pressure, the lack of appreciable pressure or the lackof a pressure increase. An obstruction might result in higher thannormal pressure or a slower than normal pressure decay.

In one embodiment, the present invention provides an implantable medicaldevice for delivering a therapeutic substance to a delivery site in apatient. A reservoir holds a supply of the fluid therapeutic substance.A catheter has a proximal end, a delivery region and a lumen extendingfrom the proximal end to the delivery region. The proximal end of thecatheter is operatively coupled to the reservoir. The delivery region ofthe catheter is adapted to be placed proximate the delivery site in thepatient. The therapeutic substance is adapted to be delivered throughthe lumen to the patient. A sensing device is operatively coupled withthe lumen of the catheter being capable of detecting a pressure of thetherapeutic substance in the lumen. A controller is operatively coupledto the sensing device, the controller being capable of taking an actionin response to the pressure in the lumen.

In another embodiment, the present invention provides a method ofdelivering a therapeutic substance to a delivery site in a patient. Thetherapeutic substance is pumped under pressure from a reservoir througha catheter fluidly coupled to the reservoir. The catheter has a proximalend, a delivery region and a lumen extending between the proximal endand the delivery region. The delivery region of the catheter is placedin proximity to the delivery site in the patient. A pressure of thetherapeutic substance is detected in the lumen. An action is taken inresponse to the pressure in the lumen.

In a preferred embodiment, the therapeutic substance is a fluid.

In a preferred embodiment, the therapeutic substance is a liquid.

In another embodiment, the present invention provides a drug deliverysystem for delivering a liquid therapeutic substance to a delivery sitein a patient. An implantable medical device has a reservoir holding asupply of the fluid therapeutic substance and a pump fluidly coupled tothe reservoir, the pump being capable of fluidly driving the therapeuticsubstance to the delivery site under pressure. A catheter has a proximalend, a delivery region and a lumen extending from the proximate end tothe delivery region. The proximal end of the catheter is operativelycoupled to the pump. The delivery region of the catheter is adapted tobe placed proximate the delivery site in the patient. The therapeuticsubstance is adapted to be delivered through the lumen to the patient. Asensing device is operatively coupled with the lumen of the catheterbeing capable of detecting a pressure of the therapeutic substance inthe lumen. A controller is operatively coupled to the sensing device,the controller being capable of taking an action in response to thepressure in the lumen.

In a preferred embodiment, the sensing device is further capable ofdetecting a reference pressure outside of the lumen and the controlleris capable of taking action in response to relative pressures betweenthe pressure in the lumen and the reference pressure outside of thelumen.

In a preferred embodiment, the location outside of the lumen is inproximity to the implantable medical device

In a preferred embodiment, the location outside of the lumen is inproximity to the delivery region of the catheter.

In a preferred embodiment, the catheter has a second lumen and thelocation outside of the first lumen is in the second lumen.

In a preferred embodiment, the location outside of the lumen is outsideof the patient.

In a preferred embodiment, action is taken when the pressure exceeds apredetermined level,

In a preferred embodiment, action is based upon an obstruction in thelumen.

In a preferred embodiment, the lumen of the catheter has a restrictionand the sensing device is positioned between the reservoir and therestriction.

In a preferred embodiment, action is taken when the pressure drops belowa predetermined level.

In a preferred embodiment, action is taken when the pressure has acharacteristic signature.

In a preferred embodiment, the characteristic signature follows atransient in delivery rate of the therapeutic substance.

Further, it can be considerably difficult to detect pressure anomaliesin catheter malfunctions because the pressure differences are relativelysmall. For example, the pressure irregularities in a cathetermalfunction can be smaller than normal pressure changes due to a changein elevation, e.g., changes in relative elevation as from lying down tostanding up. There exists a need for a highly accurate pressure sensorcapable of detecting very small pressure differences.

In another embodiment, the present invention provides a sensor for amedical device for detecting pressure in a lumen of a catheter. A basehas a first side and a second side. A first pressure sensing diaphragmis operatively coupled to the first side of the base. A second pressuresensing diaphragm is operatively coupled to the second side of the base.A connector having a first end is operatively coupled to a movableportion of the first pressure sensing diaphragm and a second end isoperatively coupled to a movable portion the second pressure sensingdiaphragm. The connector moves with the first pressure sensing diaphragmand the second pressure sensing diaphragm in response to a change inpressure. An electrical element is responsive to movement of the firstpressure sensing diaphragm and the second pressure sensing diaphragm.

In a preferred embodiment, the electrical element is both a firstelectrical sensor producing a first output in response to movement ofthe first pressure sensing diaphragm and a second electrical sensorproducing a second output in response to movement of the second pressuresensing diaphragm. The first output and the second output are combinedto produce a pressure output.

In a preferred embodiment, the first electrical sensor is a firstcapacitor and wherein the second electrical sensor comprises a secondcapacitor, wherein a capacitance of the first capacitor is a function ofa displacement of the first diaphragm and wherein a capacitance of thesecond capacitor is a function of a displacement of the seconddiaphragm.

In a preferred embodiment, a first electrical sensor is a firstsecondary coil and a second electrical sensor is a second secondarysoil. A fluctuating electrical current is induced in a primary coil. Thecurrent induced in the first secondary coil and in the second secondarycoil by inductive coupling from the primary coil is proportional to aposition of the magnetic element which in turn is a function of adisplacement of the first diaphragm and the second diaphragm

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implantable medical device in accordance with anembodiment of the present invention;

FIG. 2 is a block diagram the implantable medical device of FIG. 1;

FIG. 3 is a block diagram of a catheter containing a restriction inaccordance with a preferred embodiment of the present invention;

FIG. 4 is a drawing illustrating an exterior view of a drug deliverysystem of an embodiment of the invention;

FIG. 5 is a drawing illustrating a pressure sensor of an embodiment ofthe invention;

FIG. 6 is a drawing illustrating a cross section of the pressure sensorof FIG. 5;

FIG. 7A is a drawing illustrating a cross section of an alternativepressure sensor of an embodiment of the invention;

FIG. 7B is a schematic illustrating a circuit diagram of an LVDTpickoff;

FIG. 8 is a graph of relative catheter pressure versus time illustratingalert pressures and pressure limits according to an embodiment of theinvention;

FIG. 9 is a flow chart illustrating how pressure may be used to controla pump's delivery of a therapeutic substance;

FIG. 10A is a prior art drug delivery system without a pressure sensor;

FIG. 10B is a drug delivery system with a pressure sensor in accordancewith an embodiment of the invention; and

FIG. 11 is drawing illustrating coupling of sensors and/or sensors toelectronics according to an embodiment of the invention;

FIG. 12 illustrates a preferred embodiment of a capacitive pressuresensor; and

FIG. 13 illustrates a preferred embodiment of an inductive pressuresensor.

DETAILED DESCRIPTION OF THE INVENTION

The entire contents of U.S. application Ser. No. 10/836,115, filed Apr.30, 2004, is hereby incorporated by reference.

FIG. 1 shows implantable medical device 16, for example, a drug pump,implanted in patient 18. The implantable medical device 16 is typicallyimplanted by a surgeon in a sterile surgical procedure performed underlocal, regional, or general anesthesia. Before implanting the medicaldevice 16, a catheter 22 is typically implanted with the distal endposition at a desired therapeutic delivery site 23 and the proximal endtunneled under the skin to the location where the medical device 16 isto be implanted. Catheter 22 may disgorge therapeutic substance at otherthan at its distal end. For example, catheter 22 may intentionally havea delivery region that is not proximate its distal, e.g., a hole orvalve positioned somewhere before reaching the distal end of thecatheter 22. Thus, catheter 22 may be placed in patient 18 with adelivery region of catheter 22 placed in or near to, generally proximateto, delivery site 23.

Implantable medical device 16 is generally implanted subcutaneously atdepths, depending upon application and device 16, of from 1 centimeter(0.4 inches) to 2.5 centimeters (1 inch) where there is sufficienttissue to support the implanted system. Once medical device 16 isimplanted into the patient 18, the incision can be sutured closed andmedical device 16 can begin operation.

Implantable medical device 16 operates to infuse a therapeutic substanceinto patient 18 through catheter 22 to delivery site 23. Implantablemedical device 16 can be used for a wide variety of therapies such aspain, spasticity, cancer, and many other medical conditions.

The therapeutic substance contained in implantable medical device 16 isa substance intended to have a therapeutic effect such as pharmaceuticalcompositions, genetic materials, biologics, and other substances.Pharmaceutical compositions are chemical formulations intended to have atherapeutic effect such as intrathecal antispasmodics, pain medications,chemotherapeutic agents, and the like. Pharmaceutical compositions areoften configured to function in an implanted environment withcharacteristics such as stability at body temperature to retaintherapeutic qualities, concentration to reduce the frequency ofreplenishment, and the like. Genetic materials are substances intendedto have a direct or indirect genetic therapeutic effect such as geneticvectors, genetic regulator elements, genetic structural elements, DNA,and the like. Biologics are substances that are living matter or derivedfrom living matter intended to have a therapeutic effect such as stemcells, platelets, hormones, biologically produced chemicals, and thelike. Other substances may or may not be intended to have a therapeuticeffect and are not easily classified such as saline solution,fluoroscopy agents, disease diagnostic agents and the like. Unlessotherwise noted in the following paragraphs, a drug is synonymous withany therapeutic, diagnostic, or other substance that is delivered by theimplantable infusion device.

If catheter 22 malfunctions, i.e., has or develops a leak or anobstruction, that malfunction may be detected by analyzing the pressureof the therapeutic substance, typically a fluid and more typically aliquid, in a lumen of catheter 22. FIG. 2 illustrates, in block diagramform, an implantable medical device 16. Implantable medical device isalso shown in FIG. 10A and FIG. 10B. Therapeutic substance is stored inreservoir 24 in housing 26. Pump 28 is fluidly coupled to reservoir 24gaining access to therapeutic substance. The output of pump 28 iscoupled to catheter 22 through a check valve 30. Pump 28 and check valve30 are controllable by electronics module 32. Pressure sensor 34 isoperatively coupled to detect/sense pressure in a lumen of catheter 22.If the pressure sensed by pressure sensor 34 is not appropriate, thenelectronics module 32 may take appropriate action such as by soundingalarm 36. Refill port (see FIG. 4) may be used to refill reservoir 24without explanting implantable medical device 16.

To detect pressure within catheter 22, pressure sensor 34 may be placedin fluid contact with a lumen of catheter 22. Pressure sensor 34 may beplaced in fluid contact with a lumen of a catheter 22 anywhere along thelumen of the catheter 22. In an embodiment, where catheter 22 is coupledto implantable pump 28, pressure sensor 34 may be contained withinhousing 26. Pressure sensor 34 could also be located external to housing26. Pressure sensor 34 may be coupled to electronics module 32. For easeof coupling pressure sensor 34 to electronics module 32, it may bepreferred to locate pressure sensor 34 within housing 26. Electronicsmodule 32 may also be located in housing 26. Electronics module 32 maycontrol pump 28 and may be coupled to pressure sensor 34. Electronicsmodule 32 may stop pump 28 from continuing to deliver therapeuticsubstance when a predetermined pressure is detected in catheter 22,which pressure is indicative of a leak in or obstruction of catheter 22.

In certain circumstances it may be desirable to obtain a relativepressure within lumen of catheter 22. That is, it may be preferable tocompare a pressure within catheter 22 to a pressure not in catheter 22to avoid false indications of a leaky or obstructed catheter 22. Forexample, a pressure reading falsely indicating a leaky catheter may beobtained when catheter 22 is subjected to decreasing atmosphericpressure (e.g., when a subject having an implanted catheter with apressure sensor goes up an elevator or flies in an airplane). Similarly,a pressure reading falsely indicating that an obstruction exists withincatheter 22 may result when catheter 22 is subject to increasingatmospheric pressure (e.g., when a subject having an implanted catheterwith a pressure sensor goes scuba diving). To avoid false indications ofan obstructed or leaky catheter, it may be desirable to compare apressure within catheter 22 to a pressure not within catheter 22,preferably within the vicinity of catheter 22. Thus, the pressure notwithin catheter 22 may be used as a reference pressure.

A reference pressure may be detected within a patient's 18 body in whichcatheter 22 is implanted or may be detected outside of patient's 18body. When detected within a patient's body, a reference pressure may bedetected in a location near catheter 22 or a location in a separate areaof the patient's 18 body. A reference pressure may be obtained in anylocation capable of providing a pressure indicative of the externalenvironment of implanted catheter 22.

For example, a reference pressure may be taken in or around (in theproximity of) implantable medical device 16, i.e., in a region of asubject's body cavity where a pump system is implanted. This locationmay be preferred because the reference pressure may be taken withouttransporting (either before or after measurement) the pressure from adistant location back to implantable medical device 16. For example,FIG. 4 illustrates an implantable pump system having a housing 26 with avent 38 through which a reference pressure may be obtained.

It is preferred to take a reference in or around (in the proximity of orin the proximate area) delivery site 23. This is the best referencelocation because therapeutic substance is to be dispensed at thislocation. Any elevation difference between the delivery site 23 and thereference location would be eliminated by having the reference locationat the delivery site 23. Of course, a reference pressure could be takenoutside of the patient 18. This may be preferred, for example, whenimplantable medical device 16 reports the pressure taken in catheter toan external device for adjustment to relative and, perhaps, subsequentappropriate action. This location would eliminate the need for animplanted pressure sensor for a relative pressure measurement and wouldstill account for changes in atmospheric pressure.

Alternatively, a drug delivery system (implantable medical device 16)contains catheter 22 having a lumen for delivering a pharmacologicalagent (therapeutic substance) and a second lumen through which nopharmacological agent (therapeutic substance) is delivered. A referencepressure may then be detected in the second lumen. The second lumen incatheter 22 can easily be used to obtain a reference pressure from adistal end of catheter 22, from a delivery region of catheter 22 and/orfrom delivery site 23.

Any means capable of comparing an intracatheter pressure to a referencepressure may be used. Overviews of how such a comparison may be made areshown in FIG. 11. For example, a first sensor 42 for detecting anintracatheter pressure may be coupled to a second sensor 44 fordetecting a reference pressure. The coupled first and second sensors (42and 44) may then be coupled to electronics 46 that may interpret asignal regarding the compared pressures from the coupled first andsecond sensors (42 and 44). Electronics 46 may compare the pressures.The first and second sensors (42 and 44) may communicate with theelectronics 46 through electrical means or through other means, e.g.,telemetry. As shown in FIG. 1, the electronics 46 may be part ofelectronics module 32 and contained within housing 26. When thereference pressure is detected in an area of the body away from thecatheter 22 or external to the body, telemetry is the preferred means ofcommunicating the reference pressure to the electronics 46.

If catheter 22 has a leak, it is difficult to detect because the backpressure against a normally flowing catheter is not very high.Therefore, detecting an even lower pressure indicative of a leak isextremely difficult. It may be preferred to introduce a partialrestriction into catheter 22 in order to create a higher back pressurethan would otherwise be encountered. The partial restriction would, ofcourse, significantly limit the delivery of therapeutic substance todelivery site 23. Pressure sensor 34 is placed between pump 28 andrestrictor 48 in order to be able to detect the increased back pressure.If catheter 22 then has or develops a leak before the location ofrestrictor 48, a significant pressure drop or lack of pressure rise in atransient condition can be detected and a leak can more easily bedetected.

Preferably, catheter 22 of implantable medical device 16 containsrestriction 48 (see FIG. 3) to create back pressure within a lumen ofcatheter 22. Flow restrictor 48 may be placed in catheter 22 to impedethe flow of fluid therapeutic substance through catheter 22. Pressuresensor 34 is in fluid communication with a lumen of catheter 22 butupstream of flow restrictor 48, i.e., between pump 28 and restrictor 48,may sense backpressure within catheter 22 resulting from restrictor 48.Creating backpressure in catheter 22 may be desirable when, for example,a leak in catheter 22 is to be detected. When back pressure is createddue to restrictor 48, a leak in catheter 22 will result in a moresubstantial drop in intracatheter pressure than when no restrictor ispresent. Thus, a leak may be more easily and accurately detected whenbackpressure is created in the catheter.

Restrictor 48 may be any restrictor capable of creating backpressurewithin catheter 22 while allowing sufficient amounts of therapeuticsubstance (pharmacological agent) to be delivered from catheter 22.Examples of suitable flow restrictors include a valve, a tortuous path,and a permeable membrane. A preferred restrictor 48 is shown anddescribed in co-pending U.S. Provisional Patent Application, filed Apr.22, 2004, entitled CATHETER SYSTEM HAVING FLOW RESTRICTION ANDDIAGNOSTIC SYSTEM FOR USE WITH SAME (Attorney Docket No. 134.02180160),which is hereby incorporated by reference.

Any means for detecting pressure within a catheter or a referencepressure may be used One suitable means for detecting pressure is adiaphragm.

FIG. 5 shows an overview of an exterior view of a pressure sensor 34,where a first sensor 42 and a second sensor 44 are housed within asensor casing 50. The first sensor 42, which is adapted to detect apressure within a catheter 22, is attached to the sensor casing 50 in amanner such that no or minimal fluid from the lumen of the catheter 22may penetrate to the interior of the casing 50. The second sensor 44,which is adapted to detect a reference pressure, is also attached to thesensor casing 50 in a manner such that no or minimal fluid from maypenetrate the interior of the casing 50 from the second sensor 44. Thesensor casing 50 may contain an opening 52 such that wires or otherobjects capable of carrying a signal to electronics 46 may exit andenter the casing 50. As shown in FIG. 4, housing 26 of implantablemedical device 16 may contain a vent 38 through which the second sensor44 may detect a pressure in a body cavity of patient 18 in whichimplantable medical device 16 is implanted.

FIG. 6 illustrates a cross section of a pressure sensor 34 of FIG. 5.First sensor 42 may be coupled through physical coupler 54 to a secondsensor 44. The first and second sensors (42 and 44) may be diaphragms.

The coupler 54 and a portion of the diaphragms may move in relation to achange in pressure (reference or intracatheter). The relative movementof the coupler 54 or a diaphragm may be used to transmit informationregarding a relative intracatheter pressure. The coupler 54 may beplaced in contact with the first sensor 42 or second sensor 44 at anylocation capable of transmitting a pressure signal. When the first andsecond sensors (42 and 44) are diaphragms, it is preferred that thecoupler 54 contact the diaphragms at or near the center of thediaphragms. It is also preferred that the area of the contact betweenthe coupler 54 and the diaphragms is small to avoid stiffening of thediaphragm and to avoid potential gravitational influences.

FIG. 7A shows a sensor system 34 where a pick off means 56 is used torelay information to electronics 46 regarding a relative intracatheterpressure. A pick off means 56 may be placed anywhere in the sensorsystem 34 where the pickoff means 56 may detect a relative intracatheterpressure. For example the pick off means 56 may be placed on a sensordiaphragm or on a coupler 54. Any pick off means 56 capable of detectinga relative intracatheter pressure where first and second pressuresensors (42 and 44) are attached to a coupler 54 may be used.. Suitablepick off means 56 include optical, strain, inductive (such as LVDT),capacitive, ultrasound, etc. An exemplary circuit diagram of a LVDT pickoff 56 according to an embodiment of the invention is shown in FIG. 7B.Such a dual sensor 34 may simultaneously account for a referencepressure by moving in relation to a reference pressure.

Alternatively, a combination of a dual sensor incorporating a referencesensor and a mathematical adjustment of the measured pressure may beused. In this case, the referenced pressure may not be best referencepressure, or the best reference location. If a better reference pressureis available from another source, the relative pressure measured can bemathematically adjusted against the better reference.

FIG. 10A and FIG. 10B shows a modification to an implantable pump system(implantable medical device 16) that may be made to accommodate apressure sensor 34. FIG. 10A illustrates an implantable pump system 16without a pressure sensor 34. FIG. 10B illustrates an implantable pumpsystem 16 with a pressure sensor system 34. With the pressure sensorsystem 34, the housing 26 has a bulge 58. The bulge 58 results due to anaccommodation made to fit the pressure sensor 34 within the housing 26.The pressure sensor 34 may be placed in an area of the housing 26 suchthat it is in close proximity to electronics module 32.

In various embodiments, the invention provides methods and systems forcontrolling a pump 28 and presenting an alarm if an intracatheterpressure indicative of an obstructed or leaky catheter 22 is detected.An intracatheter or relative intracatheter pressure indicative of a leak(lower limit 60 in FIG. 8) or an obstruction (upper limit 62 in FIG. 8)or pressures nearing such limits (alert pressures 64 in FIG. 8)transmitted from a sensor or sensor system 34 to electronics 46 may beused to stop the pump 28 or sound an alert 36. When a pressure limit (60or 62) is detected, the pump 28 is stopped, or alternatively, thedelivery rate of pump 28 may be reduced. It may be desirable to reducethe delivery rate, rather than stop pump 28, in various situations, forexample when pump 28 is driving fluid through a bifurcated catheter andonly one of the bifurcated lumens is obstructed. When an alert pressure,which may be a pressure limit (60 or 62), is detected an alert isissued. An alert may comprise a warning to a patient, calling an EMT,alarming a caregiver, etc.

Limits and alert pressures may be determined by a pump or cathetermanufacture, or they may be determined by a caregiver, such as aphysician, as experience dictates. Upper limits may be, for example, themaximum pressure an implantable pump may be capable of handling. Lowerlimits may be, for example, essentially zero intracatheter pressure.

It may be preferable to introduce a transient, or a change in the rateof delivery of therapeutic substance. Such a transient can be anincrease or a decrease in the delivery rate but, typically, thepreferred transient is an increase in the delivery rate. An example of atransient in the delivery rate is a commonly occurring bolus, whetherprogrammed or initiated under patient control. Alternatively, such atransient could be an intentional introduced and specific change indelivery rate designed to more easily detect a catheter abnormality.Such a transient could be a dramatic increase in delivery rate but onlyfor a short period of time. While typically boluses may last for manyminutes or hours, such a transient may last only seconds or a fewminutes. Such a transient wouldn't substantially change the overalldosage of therapeutic substance delivered to patient 18.

Implantable medical device 16 may look for a characteristic signaturefrom pressure sensor 34 upon initiation of, during or following atransient in the delivery rate. It would be characteristic of a normalcatheter for the pressure to increase upon and shortly following anincrease in the delivery rate. It would also be characteristic of anormal catheter for the pressure to decrease over a decay time upon andfollowing a decrease in delivery rate.

If the delivery rate is increased and the pressure does notcorrespondingly increase, the signature would be indicative of a leak incatheter 22. Contrarily, if the delivery rate is decreased and thepressure does not decay, the signature would be indicative of anobstruction in catheter 22. A higher than normal decay rate would beindicative of a slight leak in catheter 22. A slower than normal decayrate would be indicative of a slight or partial obstruction of catheter22.

Pump 28 can be a peristaltic pump which operates with a plurality ofrollers squeezing a tube containing therapeutic substance. It may be acharacteristic signature of pressure readings from a catheter coupled toa peristaltic pump to have the pressure dip slightly as each of theplurality of rollers releases the tubing. This characteristic signatureof pressure in catheter should occur in a normally functioning systemusing a peristaltic pump. If this signature is absent, it is indicativeof a malfunction.

Further, the pressure occurring as each roller of a peristaltic pumpreleases tubing can be considered to be a transient inducing expectedtransient conditions in catheter 22 as discussed above.

FIG. 12 is a partial cross-sectional view of a preferred embodiment ofpressure sensor 34. Pressure sensor 34 in FIG. 12 is a capacitive flowsensor utilizing two diaphragms (66 and 68). Upper diaphragm 66 ismounted to sensor casing 50. Upper diaphragm 66 is made from or iscoated, at least partially, with a conductive material 70. Complementaryconductive material 72 is coated on stationary sapphire insulator 74. Ina preferred embodiment, a 0.002 inch gap is created between conductivematerials 70 and 72. Using air as an insulator, conductive materials 70and 72 form a capacitor. As upper diaphragm 66 moves in response topressure changes, the capacitance created by conductive materials 70 and72 also changes. A similar arrangement exists on the opposite end ofsensor casing 50 with lower diaphragm 68. Conductive materials 76 and 78are coated on lower diaphragm 68 and sapphire insulator 80,respectively, forming another capacitor. Coupler 54 is positioned forrelative movement with, preferably against, both upper diaphragm 66 andlower diaphragm 68. Capacitive sensor 82 is sensitive to changes in bothcapacitances and provides the sensing output of sensor 34 illustrated inFIG. 12. Capacitive sensor 82 is conventional. A preferredimplementation for capacitive sensor 82 is described in U.S. Pat. No.5,535,752, Halperin et al, Implantable Capacitive Absolute Pressure andTemperature Monitor System, the contents of which are herebyincorporated by reference. It is worth noting that the above-describedarrangement of dual diaphragms and dual capacitors actually multipliesthe amount of change in capacitance with a given amount of movement indiaphragms 66 and 68. Since the pressure changes are small, the movementof diaphragms 66 and 68 are small. The capacitance change is additiveresulting in twice the performance. It is preferred that coupler 54contact diaphragms 66 and 68 in the center of the diaphragms in order toobtain the maximum movement of diaphragms 66 and 68. Coupler 54 shouldnot significantly inhibit the movement of diaphragms 66 and 68.

FIG. 13 illustrates an alternative embodiment of pressure sensor 34which operates on a change in inductance. Again, sensor 34 has twodiaphragms 66 and 68 with coupler 54 mounted for movement between them.A center primary coil 84 is excited with an alternative current such asa sine wave. Upper and lower secondary coils, 86 and 88 respectively,are mounted above and below primary coil 84, respectively. Magneticelement 90 is mounted for movement with coupler 54. As magnetic element90 moves up and/or down in response to changes in pressure, theinductance induced in secondary coils 86 and 88 varies. An inductancesensor (not shown) can detect the change in these inductances andprovide an output indicative of a change in pressure. Again, thisarrangement doubles the effectiveness of movement in diaphragms 66 and68 by additively combining the changes in inductance of each individualsecondary coil (86 or 88).

The following discussion is intended to put some of the foregoingdescription in context with real world operating conditions. It is to berecognized and understood that the specific conditions, constructionsand operations discussed herein may or may not be preferred and aremerely indicative of a possible example of a construction, use,operating condition and the like and should not be considered to limitthe scope of the invention in any way.

A pressure sensor may be placed in a drug flow path of an implantabledrug delivery system, which includes a pump and a catheter. Pressuredetected at a chosen location of the pressure sensor could indicatecatheter complications such as blockage or leakage of the catheter. Forpurpose of this discussion, the sensor is assumed to be placed in acasing of a drug pump, and, preferably, a flow restriction is placed atthe tip of the catheter. However, it will be recognized that the sensormay be placed anywhere in communication with the drug flow path and thatthe flow restriction, if desired, may be placed anywhere downstream ofthe sensor. Normal operation of the pump and catheter would produce adetectable backpressure over all drug delivery flow rates. A cutcatheter would cause the pressure in the catheter to drop to of aboutzero (vs. the pressure in the fluid outside the catheter near the cut),and a blocked catheter would cause the catheter backpressure to increasesignificantly relative to cerebiospinal fluid (CSF) pressure, as anexample of a bodily location as a delivery site. A second assumption isthat the pressure drop at the catheter-tip or delivery regionrestriction is large enough relative to other pressure changes betweenthe tip or region of the catheter and the sensor that other pressurechanges affecting the pressure sensor reading will be insignificant incomparison, thus providing a robust measure of catheter status. Thus thepurpose of this analysis is to determine, for different catheterpressure measurement approaches, what the minimum pressure change wouldneed to be to distinguish catheter complications from background noisein the pressure signal.

The amount of pressure drop across a catheter-tip flow restriction willdepend to a great extent on the pressure reference which is implemented,i.e., what is the pressure in the catheter compared against. Laboratorytests of a catheter pressure diagnostic have used a differentialpressure sensor comparing catheter backpressure against atmosphericpressure. This so-called “gage” pressure reading provides a robustcomparison of pressure across the flow restriction, which automaticallycancels out potential reference pressure error sources such asatmospheric pressure variation (upon which physiologic pressures float).An atmospheric reference may not always be feasible in an implantabledevice. While ways exist to achieve an atmospheric reference with animplantable device such as plumbing a vent line across the cutaneousboundary, it may be desirable to implement another method. Thisdiscussion will look at implementations using a couple types of in-vivopressure references, and also consider the case where there is a fixedreference, either by using a vacuum reference or a reference consistingof a sealed cavity of gas.

Physiological pressure variation: In considering catheter back pressureas a correlate for catheter flow status, typical pressure variation inthe body should be taken into account. Physiologic fluid pressurestypically ride on top of atmospheric pressure.

Atmospheric pressure can vary for a number of reasons.

Altitude is probably the most significant cause of physiologicalabsolute pressure variation. Atmospheric pressure declines byapproximately 1″ Hg (about ½ psi) per 1000 feet increase in altitude.Putting it in perspective, one can expect about ¼ psi change in airpressure going from ground floor to the top floor of the IDS building indowntown Minneapolis (The IDS Center building is approximately 775 feettall, with about 57 stories). Aircraft cabins are typically pressurizedto 6000 feet above sea level, although unpressurized commuter andcharter aircraft typically fly to 10,000 feet above sea level. Thus, theambient pressure variation during a flight from a sea level airportcould be as high as 5 psi in the case of the unpressurized aircraft witha 10,000 foot cruise altitude. Ten thousand feet is also the altitude ofthe highest mountain passes in the American Rocky Mountains. Travelersin mountainous regions can also experience significant variations inatmospheric pressure.

Weather is another cause of physiological pressure variation. At a givenlocation, weather-induced atmospheric pressure variation can be on theorder of 20 mm Hg (0.1 psi).

Cerebrospinal fluid (CSF) pressure, which for purpose of this discussionis the pressure reading of interest, is roughly equal to venous bloodpressure, which in healthy individuals is around 0 psi at the level ofthe heart. However, pressure within the CSF volume can vary for a numberof reasons.

Pressure may vary due to fluid column height. Pressure in the spinalcolumn while standing, being around zero psig at the level of the heart,increases as one moves lower along the spinal column. The increasecorresponds to the height of the column of salt water between the pointof interest and the heart level. Typical variation is 0.04 psi/inch. Thepressure difference between two points is naturally affected by theposture, with a standing posture producing maximum pressure gradientalong the spinal column, and a supine posture producing minimal or zeropressure gradient.

Pressure may also vary due to disease state. Diseases such as congestiveheart failure (CHF) and hydrocephalus can cause increase in CSF pressureof up to 100 mmHg (0.05 psi).

Pressure may also vary due to transient events. Straining, coughing, andother such actions on the part of the patient can cause momentaryincreases in physiologic pressures of up to 100 mmHg (0.05 psi).Submersion in water can cause external pressure change (and subsequentphysiological fluid pressure change) on the order of 0.89 in-Hg (0.44psi) per foot of submersion. Transient pressure events can be filteredout using algorithms commonly used to disregard outlying data, and thuswill not be considered as part of the range requirements for the sensor.

These and other pressure variations may affect a pressure within acatheter. In addition, the location of a catheter within the body mayaffect pressure within the catheter. It will be recognized that thecatheter may be placed at any location in a body where delivery of apharmacological agent is desired and is not limited to positioning fordelivery of an agent to the CSF.

Diagnostic catheter pressure change elements according to an embodimentof the invention. In an absolute pressure system, pressure in thecatheter is referenced to pressure inside a sealed chamber. The pressurein the reference chamber could be any value, including a vacuum. Sincethere is no reference to either atmospheric pressure or the body cavity,the sensor can detect all of the pressure changes described in section3. Range of absolute pressures detected by the sensor would include:Atmospheric pressure 10-15 psi; Weather: add 0.01 psi to top of scale,subtract 0.01 from bottom; Posture: add 0.5 psi to top end of absolutepressure range; and Disease state: add 0.05 psi to top of pressurerange.

The total absolute pressure range requirement ranges from a low end of10 psi−0.01 psi=9.99 psi to a top end of 15 psi+0.01 psi+0.5 psi+0.05psi=15.56 psi. Pressure ranges and recommended pressure change to detectcut or leaky catheter: A catheter back pressure of at least 15.56psi−9.99 psi=5.57 psi is preferably maintained in the catheter relativeto the surrounding body fluids, to decrease the chance that the catheterdiagnostic would detect a false positive due to such things as aircraftcabin pressure fluctuations, etc. Thus the absolute pressure in thecatheter during normal operation may be as high as 15.56 psi+5.57psi=21.13 psi. For the blocked catheter diagnostic, the pump wouldpreferably deliver absolute pumping pressure higher than 21.13 psi todecrease the likelihood that catheter pressure would climb noticeablyhigher than normal operating pressure when blocked if high ambient fluidpressures are present.

Generally, smaller pressure changes may be used to detect a cut in acatheter if the time course of the pressure changes is taken intoaccount in a diagnostic algorithm. Since most transient pressure events(coughing, submersion in water, etc.) tend to increase pressure, andsignificant decreases in pressure (altitude change) occur slowly, anabrupt decrease in pressure may indicate a cut catheter. To detect suchan abrupt decrease in pressure, a pressure baseline is preferablyestablished prior to the catheter being cut.

Barometric Reference: A separate absolute pressure sensor may be used tosignificantly reduce the pressure change and max absolute pressurerequirements of a diagnostic pressure sensor since the largest errorterm, atmospheric pressure, could be eliminated from the band ofpressure uncertainty. A separate absolute pressure sensor may belocated, for example, either in the drug pump sensing peritonealpressure or carried by the patient and sensing atmospheric.

An absolute pressure sensor implementation is believed to be the moststraightforward and least disruptive in terms of amount of modificationto the pump. A sensor similar to Medtronic, Inc.'s Chronicle™ pressuresensor could be inserted in the drug path, with electrical interfacecreated to interface to the pump electronics. Adding a second sensor inthe implant for purpose of subtracting peritoneal pressure may also beemployed. With use of an external barometer a type of low-power wirelessshort-medium distance communications medium is preferred.

Differential Pressure Sensor: A differential pressure sensor may beused. A differential pressure sensor may be designed to measure thedifference between pressures in two regions, typically by applying thepressure from one region to one side of a sensor diaphragm, and thepressure from the other region to the opposite side of the sensingdiaphragm. In this way the diaphragm deflection is proportional to thedifference in pressure between the two regions.

One system uses as reference a pressure in the vicinity of the pump(peritoneal pressure), and the other system uses a pressure in the CSFat the tip of the catheter.

For a reference in the peritoneum, Error terms due to atmosphericpressure variation and global physiological pressure change would beexpected to cancel. Some errors due to local physiological pressurechange may still exist, for instance if the patient lies atop the pumpand pressure increases locally due to the weight of the patient beingsupported by tissue and fluid around the pump. The magnitude of thislocal pressure may be taken into consideration. Additional pressureerror may be due to postural effects on pressure difference between thepump implant site and the tip of the catheter, which as describedpreviously could cause up to 0.5 psi max variation in differentialpressure. Thus a catheter backpressure of about 0.5 psi or greater ispreferred.

A vent port to the pump to access peritoneal pressure may be included,and internal plumbing to conduct peritoneal fluid pressure from the portto the backside of the sensor diaphragm may be included in a pumpsystem. Preferably, fluid would be in contact with both sides of thesensing diaphragm in operation of such a system. Pickoff methods ormechanical designs that allow fluid to press against both sides of adiaphragm while keeping the pickoff hardware from contacting fluids arepreferred.

For a reference at the tip of the catheter or at a delivery region ofthe catheter, it is believed that all error terms described in thisdocument would substantially cancel out. Any means may be used to locatea pressure sensor at a catheter tip. For example, a dual-lumen catheteror a separate catheter may be employed to conduct fluid pressure fromthe tip of the catheter to the sensor in the drug pump. A minimalpressure drop across catheter tip may be sufficient for detection.

A dual lumen catheter, one lumen of which terminates with a flowrestriction, is preferred. A second catheter connector port may be addedon the pump and internal plumbing to conduct CSF pressure from theconnector port to the backside of the diaphragm may be added.Preferably, a pressure sensor design that allows fluid contact with bothsides of the diaphragm is employed. Further, it may be desirable toperiodically flush the reference lumen, as it is a stagnant column ofCSF. One way to allow flushing would be to connect the reference lumento a catheter access port of an implantable pump system. A secondcatheter access port may be used to flush the main drug delivery line.Alternatively, a means for accessing both lines from a single catheteraccess port (without allowing pressure communication between the twolumens during normal pump operation) was may be used.

So far this discussion has so far assumed a constant flow rate of drugdelivery producing a long-term pressure trend, and technical issues inseparating out changes in catheter back pressure from pressure changescaused by other influences. If the drug delivery method were changedfrom a constant flow to a pulsed approach, then the time-course ofpressure changed could be used as an additional diagnostic. In thiscase, the buildup and dissipation of pressure caused by flow pulsationcould be distinguishable from other pressure changes because of itscharacteristic signature, and because the pump controls the timing ofthe pressure pulsations and thus pressure changes which do notcorrespond with programmed drug flow pulses may be largely ignored. Asystem could thus be implemented using a single absolute pressure sensorlargely ignoring long-term pressure changes in favor of concentrating onthe pressure signature in the time window of pump pulsations.

Pulsed flow would create pressure spikes instead of a constant backpressure at a given flow rate. An advantage of a pulsed delivery is thatthe pressure spike amplitude may be easier to detect than a steady-statepressure, since at a given average flow rate the instantaneous flow rateduring the pulse, assuming the pulses are spaced apart in time, will bemuch higher than if the flow rate was constant over the same timeperiod. Also, a diagnostic algorithm may look at the pressure signalonly during the pulse, decreasing the importance of a reference pressureto compare against catheter backpressure since the pressure sensor canobtain a baseline pressure reading just before the pulse. In otherwords, the pressure change caused by the pulse relative to the pressurejust before the pulse may be used. Also, the pressure decaycharacteristic after the pulse pressure peak may provide additionalinformation that a steady state pressure may not be able to provide(because the time constant of the pressure decay is defined by thecapacity of the catheter and the forward resistance to flow). A cutcatheter would exhibit a very rapid pressure decay (and in fact probablywouldn't show a substantial peak), since there would likely be little orno forward resistance. A blocked catheter may show a slow decay, or ifcompletely blocked may show no decay at all, and may just find a newsteady-state pressure level due to the increased volume of fluid tryingto fill the blocked catheter. The point along the catheter where theblockage has occurred may be predicted by noting the pressure rise for agiven volume increase. A blockage nearer the pump would decrease thecapacity of the catheter to absorb additional fluid volume behind theblockage, and thus would likely show a very large pressure increase(relative to one where the blockage further away from the pump). Ablockage at the end of the catheter would be expected conversely exhibitmuch lower pressure increase after a pulse since there is more capacityto absorb the additional volume of fluid.

Thus, embodiments of the invention are disclosed. One skilled in the artwill appreciate that the present invention can be practiced withembodiments other than those disclosed. The disclosed embodiments arepresented for purposes of illustration and not limitation, and thepresent invention is limited only by the claims that follow.

1. An implantable medical device for delivering a therapeutic substanceto a delivery site in a patient, comprising: a reservoir holding asupply of said fluid therapeutic substance; a catheter having a proximalend, a delivery region and a lumen extending from said proximal end tosaid delivery region, said proximal end of said catheter beingoperatively coupled to said reservoir, said delivery region of saidcatheter being adapted to be placed proximate said delivery site in saidpatient, said therapeutic substance adapted to be delivered through saidlumen to said patient; a sensing device operatively coupled with saidlumen of said catheter being capable of detecting a pressure of saidtherapeutic substance in said lumen; and a controller operativelycoupled to said sensing device, said controller being capable of takingan action in response to said pressure in said lumen.
 2. An implantablemedical device as in claim 1 wherein said therapeutic substancecomprises a fluid.
 3. An implantable medical device as in claim 2wherein said therapeutic substance comprises a liquid.
 4. An implantablemedical device as in claim 3 wherein said sensing device is furthercapable of detecting a reference pressure outside of said lumen andwherein said controller is capable of taking action in response torelative pressures between said pressure in said lumen and saidreference pressure outside of said lumen.
 5. An implantable medicaldevice as in claim 4 wherein said location outside of said lumen is inproximity to said implantable medical device.
 6. An implantable medicaldevice as in claim 4 wherein said location outside of said lumen is inproximity to said delivery region of said catheter.
 7. An implantablemedical device as in claim 6 wherein said lumen is a first lumen andwherein said catheter has a second lumen and wherein said locationoutside of said first lumen is in said second lumen.
 8. An implantablemedical device as in claim 4 wherein said location outside of said lumenis outside of said patient.
 9. An implantable medical device as in claim4 wherein said sensing device comprises:: a first pressure sensoroperatively coupled with said lumen of said catheter being capable ofdetecting a pressure of said therapeutic substance in said lumen; and asecond pressure sensor being capable of detecting a pressure at alocation outside of said lumen.
 10. An implantable medical device as inclaim 2 wherein said action is taken when said pressure exceeds apredetermined level.
 11. An implantable medical device as in claim 10wherein said action is based upon an obstruction in said lumen.
 12. Animplantable medical device as in claim 3 wherein said lumen of saidcatheter has a restriction and wherein said sensing device is positionedbetween said reservoir and said restriction.
 13. An implantable medicaldevice as in claim 12 wherein said action is taken when said pressuredrops below a predetermined level.
 14. An implantable medical device asin claim 13 wherein said action is based upon a leak of said catheter.15. An implantable medical device as in claim 12 wherein said action istaken when said pressure has a characteristic signature.
 16. Animplantable medical device as in claim 15 wherein said characteristicsignature follows a transient in delivery rate of said therapeuticsubstance.
 17. An implantable medical device as in claim 16 wherein saidtransient comprises a rise in said delivery rate of said therapeuticsubstance.
 18. An implantable medical device as in claim 16 wherein saidtransient comprises a fall in said delivery rate of said therapeuticsubstance.
 19. An implantable medical device as in claim 16 wherein saidtransient comprises a rise and subsequent fall in said delivery rate ofsaid therapeutic substance.
 20. An implantable medical device as inclaim 19 wherein said characteristic signature is a function of a decaytime of said pressure following said fall of said delivery rate of saidtherapeutic substance.
 21. An implantable medical device as in claim 19wherein said characteristic signature is a function of an increase insaid pressure in response to an increase of said delivery rate.
 22. Animplantable medical device as in claim 12 further comprising a pumpfluidly coupled between said reservoir and said sensing device, saidpump being capable of driving said therapeutic substance through saidcatheter under pressure.
 23. An implantable medical device as in claim22 wherein said pump comprises a peristaltic pump.
 24. An implantablemedical device as in claim 23 wherein said peristaltic pump comprises atubular structure and a plurality of roller's driving said therapeuticsubstance through said tubular structure, said peristaltic pump creatinga pressure dip each time one of said plurality of roller's lifts off ofsaid tubular structure and wherein said characteristic signaturecomprises said pressure dropping below a predetermined threshold incombination with a lack of said pressure dip.
 25. An implantable medicaldevice as in claim 24 wherein said action comprises modifying operationof said pump.
 26. An implantable medical device as in claim 1 whereinsaid action comprises ceasing delivery of said therapeutic substance tosaid patient.
 27. An implantable medical device as in claim 26 furthercomprising a pump fluidly coupled between said reservoir and saidcatheter, said pump being capable of driving said therapeutic substancethrough said catheter under pressure.
 28. An implantable medical deviceas in claim 27 wherein said action comprises modifying operation of saidpump.
 29. A drug delivery system for delivering a liquid therapeuticsubstance to a delivery site in a patient, comprising: an implantablemedical device, comprising: a reservoir holding a supply of said fluidtherapeutic substance; and a pump fluidly coupled to said reservoir,said pump being capable of fluidly driving said therapeutic substance tosaid delivery site under pressure; a catheter having a proximal end, adelivery region and a lumen extending from said proximate end to saiddelivery region, said proximal end of said catheter being operativelycoupled to said pump, said delivery region of said catheter beingadapted to be placed proximate said delivery site in said patient, saidtherapeutic substance adapted to be delivered through said lumen to saidpatient; a sensing device operatively coupled with said lumen of saidcatheter being capable of detecting a pressure of said therapeuticsubstance in said lumen; and a controller operatively coupled to saidsensing device, said controller being capable of taking an action inresponse to said pressure in said lumen.
 30. A drug delivery system asin claim 29 wherein said sensing device is further capable of detectinga reference pressure outside of said lumen and wherein said controlleris capable of taking action in response to relative pressures betweensaid pressure in said lumen and said reference pressure outside of saidlumen.
 31. A drug delivery system as in claim 30 wherein said locationoutside of said lumen is in proximity to said implantable medicaldevice.
 32. A drug delivery system as in claim 30 wherein said locationoutside of said lumen is in proximity to said delivery region of saidcatheter.
 33. A drug delivery system as in claim 32 wherein said lumenis a first lumen and wherein said catheter has a second lumen andwherein said location outside of said first lumen is in said secondlumen.
 34. A drug delivery system as in claim 30 wherein said locationoutside of said lumen is outside of said patient.
 35. A drug deliverysystem as in claim 30 wherein said sensing device comprises: a firstpressure sensor operatively coupled with said lumen of said catheterbeing capable of detecting a pressure of said therapeutic substance insaid lumen; and a second pressure sensor being capable of detecting apressure at a location outside of said lumen.
 36. A drug delivery systemas in claim 29 wherein said action is taken when said pressure exceeds apredetermined level.
 37. A drug delivery system as in claim 36 whereinsaid action is based upon an obstruction in said lumen.
 38. A drugdelivery system as in claim 29 wherein said lumen of said catheter has arestriction and wherein said sensing device is positioned between saidreservoir and said restriction.
 39. A drug delivery system as in claim38 wherein said restriction is located near said distal end of the saidcatheter.
 40. A drug delivery system as in claim 38 wherein said actionis taken when said pressure drops below a predetermined level.
 41. Adrug delivery system as in claim 40 wherein said action is based upon aleak of said catheter.
 42. A drug delivery system as in claim 41 whereinsaid action is taken when said pressure has a characteristic signature.43. A drug delivery system as in claim 42 wherein said characteristicsignature follows a transient in delivery rate of said therapeuticsubstance.
 44. A drug delivery system as in claim 43 wherein saidtransient comprises a rise in said delivery rate of said therapeuticsubstance.
 45. A drug delivery system as in claim 43 wherein saidtransient comprises a fall in said delivery rate of said therapeuticsubstance.
 46. A drug delivery system as in claim 43 wherein saidtransient comprises a rise and subsequent fall in said delivery rate ofsaid therapeutic substance.
 47. A drug delivery system as in claim 46wherein said characteristic signature is a function of a decay time ofsaid pressure following said fall of said delivery rate of saidtherapeutic substance.
 48. A drug delivery system as in claim 46 whereinsaid characteristic signature is a function of an increase in saidpressure in response to an increase of said delivery rate.
 49. A drugdelivery system as in claim 29 wherein said pump comprises a peristalticpump.
 50. A drug delivery system as in claim 49 wherein said peristalticpump comprises a tubular structure and a plurality of rollers drivingsaid therapeutic substance through said tubular structure, saidperistaltic pump creating a pressure dip each time one of said pluralityof rollers lifts off of said tubular structure and wherein saidcharacteristic signature comprises said pressure dropping below apredetermined threshold in combination with a lack of said pressure dip.51. A drug delivery system as in claim 50 wherein said action comprisesstopping operation of said pump.
 52. A drug delivery system as in claim29 wherein said action comprises ceasing delivery of said therapeuticsubstance to said patient.
 53. A drug delivery system as in claim 52wherein said action comprises stopping operation of said pump.
 54. Amethod of delivering a therapeutic substance to a delivery site in apatient, comprising the steps of: pumping said therapeutic substanceunder pressure from a reservoir through a catheter fluidly coupled tosaid reservoir, said catheter having a proximal end, a delivery regionand a lumen extending between said proximal end and said deliveryregion, said delivery region of said catheter placed in proximity tosaid delivery site in said patient; detecting a pressure of saidtherapeutic substance in said lumen; and taking an action in response tosaid pressure in said lumen.
 55. A method as in claim 54 wherein saidtherapeutic substance comprises a fluid.
 56. A method as in claim 55wherein said therapeutic substance comprises a liquid.
 57. A method asin claim 56 said detecting step further comprises detecting a referencepressure outside of said lumen and wherein said taking an action stepfurther comprises acting in response to relative pressures between saidpressure in said lumen and said pressure outside of said lumen.
 58. Amethod as in claim 57 wherein said location outside of said lumen is inproximity to said implantable medical device.
 59. A method as in claim57 wherein said location outside of said lumen is proximity to saiddelivery region of said catheter.
 60. A method as in claim 57 whereinsaid location outside of said lumen is outside of said patient.
 61. Amethod as in claim 54 wherein said action is taken when said pressureexceeds a predetermined level.
 62. A method as in claim 61 wherein saidaction is based upon an obstruction in said lumen.
 63. A method as inclaim 54 wherein said lumen has a restriction and wherein said action istaken when said pressure drops below a predetermined level.
 64. A methodas in claim 63 wherein said action is based upon a leak of saidcatheter.
 65. A method as in claim 54 wherein said action is taken whensaid pressure has a characteristic signature.
 66. A method as in claim65 wherein said transient comprises a rise in said delivery rate of saidtherapeutic substance.
 67. A method as in claim 65 wherein saidtransient comprises a fall in said delivery rate of said therapeuticsubstance.
 68. A method as in claim 65 wherein said transient comprisesa rise and subsequent fall in said delivery rate of said therapeuticsubstance.
 69. A method as in claim 68 wherein said characteristicsignature is a function of a decay time of said pressure following saidfall of said delivery rate of said therapeutic substance.
 70. A methodas in claim 65 wherein said characteristic signature comprises saidpressure dropping below a predetermined threshold in combination with alack of a pressure dip created each time one of a plurality of rollerslifts off of a tubular- structure in a peristaltic pump.
 71. A method asin claim 54 wherein said action comprises ceasing delivery of saidtherapeutic substance to said patient.
 72. A sensor for a medical devicefor detecting pressure in a lumen of a catheter, comprising: a basehaving a first side and a second side; a first pressure sensingdiaphragm operatively coupled to said first side of said base; a secondpressure sensing diaphragm operatively coupled to said second side ofsaid base; a connector having a first end operatively coupled to amovable portion of said first pressure sensing diaphragm and a secondend operatively coupled to a movable portion said second pressuresensing diaphragm, said connector moving with said first pressuresensing diaphragm and said second pressure sensing diaphragm in responseto a change in pressure; and an electrical element responsive tomovement of said first pressure sensing diaphragm and said secondpressure sensing diaphragm.
 73. A sensor as in claim 72 wherein saidelectrical element comprises: a first electrical sensor producing afirst output in response to movement of said first pressure sensingdiaphragm; and a second electrical sensor producing a second output inresponse to movement of said second pressure sensing diaphragm; whereinsaid first output and said second output are combined to produce apressure output.
 74. A sensor as in claim 73 wherein said first outputand said second output are additive resulting in a doubled electricaloutput.
 75. A sensor as in claim 74 wherein said pressure outputcomprises a ratio of said first output and said second output.
 76. Asensor as in claim 74 wherein said pressure output comprises a dutycycle of one of said first output and said second output divided by asum of both of said first output and said second output.
 77. A sensor asin claim 74 wherein said pressure output comprises a difference of saidfirst output and said second output.
 78. A sensor as in claim 73 whereinsaid first electrical sensor comprises a first capacitor and whereinsaid second electrical sensor comprises a second capacitor, wherein acapacitance of said first capacitor is a function of a displacement ofsaid first diaphragm and wherein a capacitance of said second capacitoris a function of a displacement of said second diaphragm.
 79. A sensoras in claim 73 further comprising a primary coil positioned between saidfirst electrical sensor and said second electrical sensor, wherein saidconnector comprises a magnetic element, wherein said first electricalsensor comprises a first secondary coil and wherein said secondelectrical sensor comprises a second secondary soil, wherein afluctuating electrical current is induced in said primary coil, whereina current induced in said first secondary coil and in said secondsecondary coil by inductive coupling from said primary coil beingproportional to a position of said magnetic element which in turn is afunction of a displacement of said first diaphragm and said seconddiaphragm.